There is a receptor for C3 in the human renal glomerulus. Scanning electron microscopic studies demonstrate that this receptor is on the epithelial cell. The presence or absence of this receptor was investigated in a series of patients in whom glomerular C3 was present to a variable extent. In those renal biopsies in which large amounts of C3 were deposited in vivo in the glomeruli, The C3 receptor was not detectable or only minimally detectable, whereas in those patients in whom only small amounts of C3 were deposited in the glomeruli, the C3 receptor was present.